It is desirable to dry grain and other crops not only to reduce spoilage, but also to save on shipping charges based on weight, which otherwise would be calculated to include shipping the entire original moisture content of the grain or other agricultural product. Drying is also used to achieve a more or less standard or target moisture content, representing a regulatory or commercially desired maximum or optimum. Apart from any quality effects, it is undesirable to sell grain (and other products worth more than water) having a moisture content substantially less than such a maximum. Thus, it is common not only to reduce the moisture content of grain, but not to reduce it substantially below an acceptable maximum. This means the drying method must not only be efficient but readily controlled to achieve a target moisture content.
A rather basic method commonly used is simply to employ a large blower to force a stream of untreated atmospheric air through a silo or other container of grain. This method is subject to the vicissitudes and vagaries of weather conditions, particularly the temperature and relative humidity, and may actually add moisture to the grain rather than remove it. It is not efficient when the relative humidity is high, and generally cannot be used at night or at other times when temperatures are cool; therefore the operator may not be able to completely dry the grain in time to meet scheduled rail cars or other transportation. Also, the blowers must be quite large and will consume large quantities of power over time when relative humidity is high or when the back pressure is significant.
To increase the efficiency of atmospheric blowers, heaters for the air have been added, although simply heating does not remove moisture from the air but merely lowers the relative humidity. Some dryers using heated air also employ mechanical movers or manipulators of one type or another for the grain, so that the air need not pass through an entire bed of grain at once. If this is not done, the warm air has a tendency to deposit the moisture picked up from the lower (or upstream) part of a bed, into the upper (or downstream) part of the bed, as it is cooler than the warm air not carrying significant amounts of moisture. This means the warm air must do its job of picking up moisture more than once, an obviously inefficient result. To overcome this, the operator may increase the temperature further, which may tend to toast or at least over-dry the lower parts of the grain bed, reducing the value of the grain in more ways than one. And, the presence in the area of a flame to heat the air requires safety precautions because of the danger of explosions from grain dust. Fire hazards in such installations greatly increase insurance costs as well if insurance is available at all. Of course costs are increased by the additional equipment required for heating the air.
An early Cushing, U.S. Pat. No. 1,390,341, describes an air-tight silo having radial pipes with perforations used for the distribution of compressed air; the silo is first decompressed to create a vacuum, and the compressed air is then released into the silo, followed by the removal of moisture. The silo remains closed, however, and the compressed air is not passed through a bed of material but simply fills the silo. No means for drying the compressed air are shown. Compressed air is also used in a drying system by Element in U.S. Pat. No. 2,494,644.
In U.S. Pat. No. 4,189,848, inventors Grodzka and McCormick note that the conventional heated air techniques used for drying grain waste considerable energy, as the energy used to heat the air is released to the atmosphere after the process. Their answer is to circulate the air through a desiccant to aid in removing the moisture and they provide for the conservation of heat energy partly by recirculating the desiccant, which means dehydrating it for reuse. Desiccant is circulated also by Shoeld in U.S. Pat. No. 2,376,095.
Woodard, in U.S. Pat. No. 5,632,805, describes the assisted dehydration of compressed air through the use of various dehydrating devices, including a semipermeable membrane, interposed between compression stages in the compressor. No mention is made of using the air for drying grain or other agricultural products, nor is it suggested that the delivered air be heated for that purpose. Henis and Tripodi, in U.S. Pat. No. 4,230,463, review the history of membrane separation techniques beginning with cellulose acetate coatings for porous supports, used in desalination by reverse osmosis, and continuing with the early use of membranes for gas separation.
As summarized in the "Background" of Rice's U.S. Pat. No. 4,894,068, column 1, lines 23-36:
". . . (I)t is known to make relatively pure nitrogen from air by moving air under pressure into one end of an elongated container filled with a plurality of juxtaposed axially hollow membrane fibers running longitudinally of the container. Oxygen, carbon dioxide, water and other ("fast") gases will permeate through the membrane fibers, but nitrogen will permeate to a lesser extent. The gases passing through the membrane and separated from air are withdrawn from the downstream side of the membrane. As a result the portion of the air which does not permeate the membrane fibers after contact with the active membrane surface is relatively pure nitrogen.
The Rice '068 patent goes on to describe the use of hollow fiber membrane modules arranged serially for the introduction of air to achieve a nitrogen permeate having less than 1000 ppm oxygen.
The CACTUS.RTM. dryer manufactured by Permea, Inc., St. Louis Mo. is described by Rice et al in U.S. Pat. No. 4,783,201. As will be seen below, this device is useful in our invention. See also Brockmann et al U.S. Pat. No. 5,131,929.